1. Field of the Invention
The present invention relates to a semiconductor device using heterojunction bipolar transistors (hereinafter abbreviated to HBTs) and its fabrication method, and, more particularly, to a power amplifier featuring high power conversion efficiency even when the power supply voltage is 2 V or below and reduced cost of manufacturing thereof.
2. Description of the Prior Art
In recent years, as the demand for mobile communications equipment has been increasing rapidly, the research and development of power amplifiers for use in the communications equipment have been made actively. Semiconductor transistors that are used as the power amplifiers for the mobile communications equipment include GaAs HBTs, GaAs field-effect transistors (hereinafter abbreviated to FETs), and Si MOS (Metal Oxide Semiconductor) FETs. Among them, GaAs HBTs are the most commonly used transistors oriented to the power amplifier for the mobile communications equipment because of the features thereof: good linearity of input-output characteristics, operation on only the positive supply-voltage, for which circuits and components for generating a negative supply-voltage are not required, high output power density, less chip area for mounting, which results in compactness and reduced cost of manufacturing.
For a GaAs HBT, voltage VBE for turning its base-emitter junction on is approximately 1.4 V. If GaAs HBTs are used in a monolithic microwave integrated-circuit (hereinafter abbreviated to MMIC) including bias circuits, as is shown in FIG. 29, power supply voltage VCC of 2.8 V or above is required which is double the VBE. As of 2001, power supply voltage that is most usually used in mobile communications equipment is 3.5 V. Thus, there is no problem in using the GaAs HBTs as the power amplifier for mobile communications equipment. In future, however, the power supply voltage for mobile communications equipment is expected to decrease to 2V or below for reducing the power consumed by the digital circuits included in the equipment. Ultimately, it is inevitable that the supply voltage decreases to 1.5 V that is equivalent to the voltage supplied by a dry cell. In the coming time when lower power supply voltage for mobile communications equipment will be mainstream as described above, a problem should arise that the GaAs HBTs will become unable to be used as the transistors oriented to the power amplifier for mobile communications equipment.
Because the emitter-base turn-on voltage VBE is almost equal to the forbidden bandgap potential of the base material, in order to decrease the VBE, InGaAs or GaAsSb that is a narrow bandgap semiconductor should be used as the base material. Heretofore, the study on HBTs with the base of InGaAs (the InAs mole fraction is 0.5) was disclosed in, for example, OYO-BUTURI Vol. 66, No. 2 (1997), pp. 156–160. The study on HBTs with the base of GaAsSb (the BaAs mole fraction is 0.5) was disclosed in, for example, Journal of Vacuum Science and Technology Vol. 18, No. 2 (2000), pp. 761–764. These references reported the VBE measurements of 0.7 V and 0.6 V respectively.
According to the above two references of previous HBT techniques, InP that is lattice-matched to the base material was used as the substrate. However, the InP substrate is more expensive than the GaAs substrate on a same diameter basis and increasing the diameter of the InP substrate is more difficult than for the GaAs substrate. This posed a problem of higher cost of power amplifier manufacturing by these techniques.
In contrast, HBTs with the base of InGaAs, using the GaAs substrate that is less costly and can be made to have a larger diameter were disclosed in IEEE Electron Device Letters Vol. 21, No. 9 (2000), pp. 427–429. As is illustrated in FIG. 27, the feature of these HBTs is that a compositionally graded InGaP buffer layer 2 with a thickness of 1.5 μm exists between the InGaAs HBTs 34 and the GaAs substrate 1. The InGaP buffer layer 2 shuts up dislocation due to lattice-mismatch to the substrate in it so that the dislocation does not extend to the crystalline layers constituting the HBTs.
However, because the thermal resistivity of alloy semiconductors such as, typically, InGaP, is about 10 times as great as that of GaAs, the junction temperature of the InGaAs-base HBTs tends to rise during operation. As the junction temperature rises, the collector current increases, which further rises the junction temperature; that is, positive feedback takes place. In consequence, the so-called thermal runway that collapses the HBTs tends to occur, which results in a significant decrease in reliability of the HBTs and a semiconductor device using the HBTs. To suppress the junction temperature rise, the thickness of the InGaP buffer layer should be 0.1 μm or less, so that the increase of thermal resistivity of the buffer layer will be negligible. However, a 0.1 μm-thick buffer layer cannot prevent dislocation due to lattice-mismatch from extending to the crystalline layers constituting the HBTs. Consequently, when the HBTs are operating, dislocation propagates in the crystalline layers constituting the HBTs, especially, in the base layers, and the density in the carrier-recombination center increases. This causes another reliability problem of current-induced degradation of current gain.
In addition to the above reliability problems, the previous HBT techniques have limitations. In order to increase the power conversion efficiency of a power amplifier with a low power supply voltage, it is necessary to reduce the knee voltage (minimum collector-emitter voltage at the operating collector current density when the HBTs carry a common emitter current for operation) in the current-voltage characteristics. The knee voltage is primarily determined by the sum of the emitter resistance and the collector resistance and these resistances must be minimized. According to the previous HBT techniques, for the HBTs with the emitters up, collector electrodes 25 as illustrated in FIG. 27 are laterally formed on the sub-collector layer 3 which is a semiconductor (for the HBTs with the collectors up, the emitter electrodes instead of the collector electrodes). The collector resistance (emitter resistance) due to the series resistance of the sub-collector layer 3 hindered the knee voltage decreasing. The heretofore reported minimum measurement of the knee voltage (defined at collector current density 2×104 A/cm2) is 0.15 V.